The invention relates to a process for the simple disposal of filter dusts which are produced during the melting of inorganic compounds.
Electroremelting slags are used in the ESR process (electroslag remelting process) and serve, inter alia, for taking up certain impurities from the special steels and alloys which are to be produced. The inorganic slags used in this case are principally produced from aluminum oxide, calcium oxide and calcium fluoride compounds.
Steelworks slags likewise serve for purifying special steels, e.g. in ladle metallurgy, and as covering agents in the continuous casting process. The inorganic slags used in this case are principally produced from aluminum oxide and calcium oxide compounds.
Powdered fluxes are used in submerged arc welding (SAW process); they serve, inter alia, for protecting the welding zone against ingress of air, for improving arc stability, and for having specific metallurgical effects on the melting pool by calcining or burning off alloying elements. Powdered fluxes are principally produced from inorganic oxides and fluorides.
All of these slags and powdered fluxes are used in molten form and are produced by mixing the corresponding raw materials, which are generally of natural origin, for example with fluorite as a source of CaF2, and fusing them. Depending on the process, this can take place directly at the planned point of use of the slags, but preferably the slags and powdered fluxes are melted in a separate furnace and then the mixtures, after solidifying, are crushed to a ready-to-use grain size, since this guarantees a constant slag quality having a low content of unwanted and interfering compounds. These products are termed premelted slags and powdered fluxes. The slags and fluxes may also be prepared by remelting slags and fluxes which have been prepared previously.
During the melting of these raw materials, therefore, various impurities introduced via the raw materials escape or evaporate. These are removed by suction together with the dusts which are formed by the furnace charging, and are deposited on suitable filters. The filter dust is low in SiO2, because SiO2 is not volatile at the melt temperatures. As there are as yet no appropriate possibilities for using these filter dusts, they must be disposed of.
Above ground disposal without pretreatment of these filter dusts is generally not possible, since, as described in the comparison example, the leaching criteria prescribed by the legislature for this type of deposition are not complied with. As a result, those skilled in the art consider suitable chiefly the following alternative disposal routes:
disposal in a below-ground deposit, if the appropriate requirements are met;
vitrification of the filter dusts, i.e. the dusts are fused together with a large amount of SiO2 or products having a high SiO2 content, so that the dusts are incorporated into the glass phase. It must be noted that if insufficient SiO2 is incorporated, no glass phase can be formed;
the dusts are taken up with water by adding acid to a pH of below 7, nitrites are destroyed via an appropriate chemical treatment such as oxidation or reduction, water-soluble fluorides are bound by precipitation, for example by milk of lime, and then disposed in the wet state after filtration.
All these processes are relatively expensive, since either a subsurface disposal is required, or the mass of the substance to be disposed of is drastically increased by, for example, water or SiO2, and/or complex working steps are required.
The invention relates to a process for treating filter dusts which are produced during the melting of inorganic compounds containing fluorides and oxides, in which process the filter dusts are fused without substantial addition of SiO2 containing fluxes which would form a glass phase.
It has been found that fusing the filter dusts in the absence of SiO2 containing fluxes can decrease the eluable fluoride, nitrite, any heavy metals present, such as chromium and copper, and the water-soluble content, to such an extent that above ground disposal is possible without problems. When the filter dusts are melted, no vitreous melts are obtained.
Particular advantages of the process are that the disposal of the fused filter dusts is simple because the mass of material to be deposited is not increased, above ground deposition is possible in accordance with environmental regulations, and the process is simple to carry out.
The filter dusts can be fused in the same furnace in which the target products are produced. The melt temperature is preferably reduced by at least 100 K, in particular by at least 200 K, in comparison with the temperature required for producing the desired products. After complete fusion of the filter dusts used, the resultant melt preferably is immediately poured out of the furnace for cooling in order to reduce evaporation of volatile compounds as much as possible.
The inorganic compounds are preferably mixtures from which premelted slags and powdered fluxes are produced by melting. Preferably, the inorganic compounds are selected from aluminum oxide, calcium oxide and if appropriate other inorganic oxides in lesser amounts which are necessary to create the product properties which are required in the flux being prepared, and calcium fluoride.
In preparing the slags and fluxes, the inorganic components, depending on the particular slag or flux composition, are preferably melted at temperatures from 1400xc2x0 C. to 1800xc2x0 C., in particular at from 1500xc2x0 C. to 1700xc2x0 C. The melt is kept at this temperature for some time, in particular from one to two hours, in order to decrease the concentration of unwanted elements and compounds and in order to homogenize the mixture. At these temperatures, various impurities which were introduced via the raw materials and some of which are unwanted in the ready-to-use slags and powdered fluxes, escape or evaporate. Water-soluble fluorides, nitrites and heavy metal compounds are especially interfering. The compounds which are volatile at the process temperatures are removed by suction together with the dusts which are formed by the furnace charging and are deposited on suitable filters. The relatively high temperatures which are necessary to fuse the inorganic salts also form various nitrites in the gas phase over the melt and at the melt/air interface from nitrogen and oxygen. These nitrites likewise pass into the filter dust.